Permanent magnet electrical machines, such as a generator or motor, generally include a stator and a rotor configured to convert mechanical power to electrical power or vice versa. The stator typically includes a plurality of stator teeth configured to receive coils or windings wrapped around the outer perimeter thereof. The rotor may be an interior permanent magnet (IPM) configuration wherein slots are defined in steel (or other magnetic material) laminations, wherein a permanent magnet material is retained in the slots.
While the IPM rotor has many advantages, centrifugal loading at high rotational speeds of the rotor is a limiting factor, and the slots are typically designed with structural circumferential “bridges” sufficient to withstand the mechanical stress of such forces and prevent the permanent magnets from being dislodged. The bridges may also be necessary simply for handling and assembly of the rotor, and are otherwise unnecessary for operation of the machine.
These bridges, however, create the problem of “shorting” or leakage of magnetic flux from the permanent magnet, causing a corresponding loss in efficiency of the machine. Flux leakage occurs when lines of flux from one pole of the magnet pass through the rotor bridge material without crossing the air gap and passing through the stator. In typical IPM rotor structures, it is estimated that about 15% of the magnetic material is not utilized or wasted as a result of this flux leakage. This is significant when considering the high cost of the permanent magnet material.
Prior attempts have been made to physically reduce the size of the rotor material bridges. However, it has been generally recognized that a minimum bridge thickness is required for mechanical stress purposes, as well as for physical handling and assembly of the rotor. Another rotor construction known in the art as a “pole cap design” attempts to eliminate the bridges altogether by using non-magnetic bolting techniques to mechanically attach the laminations and magnets to the rotor. This method, however, requires numerous fasteners and is considered cost prohibitive for many designs.
Accordingly, a rotor lamination stack with decreased flux leakage without sacrifice of structural integrity would be welcomed in the technology.